The purpose of this study is to measure the intracellular ionic activities of ions in heart muscle cells. To determine how those activities are regulated and the effects of changing those activities on the electrical properties of the heart muscle cells. Emphasis will be on the measurement of Cl minus and dK ion because liquid ion-exchanger microelectrodes are available for measuring the activities of these ions. There are microelectrodes that measure other ions, e.g.Na ion and H ion, but they have larger tips than the liquid ion-exchanger microelectrodes and therefore are not suitable for routine use with heart musce. Using Cl minus selective microelectordes it has now been established that the intracellular Cl minus in dog cardiac Purkinje fibers and cat papillary muscle is not passively distributed, i.e. it is not in electrochemical equilibrium. Preliminary experiments with sheep purkinje fibers indicate that ouabain affects Cl minus distribution and these experiments will be extended to dog Purkinje fibers. Also in the sheep purkinje fiber experiments, it was found that the depolarization associated with ourbain intoxication is not caused by a decrease in the intracellular K ion. As with the Cl minus experiments these experiments these studies will be repeated and extended using dog Purkinje fibers. The reasons for changing from sheep to dog are that the geometry of the dog preparations are more suitable in that the fluid in the extracellular space exchanges more radily with the bathing solution and the sheep preparations are difficult to obtain. In both the Cl minus and K ion studies double-barrel microelectrodes will be used which will enable us to follow the intracellular activities as continuous functions of time which has not been possible with the single barrel microelectrodes.